【作者】 王亮；

【导师】 沈传波； Giacomo Corti；

【作者基本信息】 中国地质大学 ， 矿产普查与勘探， 2021， 博士

【摘要】 作为处于威尔逊旋回萌芽阶段的、仍在活动的典型陆内裂谷,新生代的东非裂谷是研究裂谷构造的理想对象,而其中位于裂谷东支中段的Turkana坳陷则因其经历了长期复杂的构造改造作用而具有特殊的研究意义。前人研究普遍认为东非裂谷受南北向展布的前寒武系基底先存构造控制,并沿古构造带发育,然而,部分地区断裂与露头上基底先存构造大角度相交,受基底先存构造控制作用不明显。近期研究认为该地区至少经历了两期伸展作用,新生代裂谷形态及演化明显受中生代裂谷活动形成的先存构造的活化作用影响。受前寒武系基底先存构造(变质岩线理和/或面理)及中生代裂谷作用的影响,研究区新生代裂谷演化过程复杂,而关于先存构造对新生代裂谷发育演化及构造形态的控制作用的认识仍不清晰。另外,研究区的地幔柱活动也是影响新生代裂谷形成演化的重要因素,关于研究区地幔活动是否、以及如何影响新生代裂谷发育演化特征的认识也不明确。本文利用大量二维地震资料及钻井资料,对新生代裂谷构造几何学和运动学进行综合分析,在此基础上,利用构造物理模拟分析手段,进一步开展以下两个问题的研究:第一,经历多伸展阶段的裂谷活动中,先存构造(先存断裂)活化的特点,以及先存构造对晚期裂谷形成演化的影响;第二,地幔活动如何影响裂谷特殊的几何形态和演化过程?地幔活动和先存构造的联合作用,如何影响裂谷演化模式和形成机制?通过大量地震资料精细解释,本文进一步深化了对东非裂谷Turkana坳陷的构造几何学和运动学认识。落实研究区7个凹陷的基本沉积充填及结构特征:研究区整体充填渐新统-更新统,受边界断裂控制,共发育典型持续型地堑、典型半地堑、迁移型半地堑、碟式不对称地堑和平行变换型地堑5种凹陷结构类型。平面上,各凹陷由南向北,地层充填由老逐渐变新,结构特征上有由“半地堑”向“地堑”转变、由“东西平行”NNW向宽裂谷向“南北带状”窄裂谷转变的特点。根据搜集到的最新的定年数据,结合部分钻井及野外露头数据,厘定裂谷活动始于Turkana坳陷中南部的South Lokichar凹陷(45Ma),之后向南北呈“跳跃式”发展。通过分析主要边界断层的断层活动速率和骨干剖面的构造发育史特征,新生代裂谷演化可分为两个期次四个演化阶段。研究区演化特点表现为:南早北晚,先宽后窄,向东迁移。即新生代早期,一系列裂谷NW-SE向近平行发育;早中新世,裂谷集中于南部South Lokichar及Kerio附近;中-中新世后,裂谷向北迁移演化;上新世以来,裂谷向东迁移演化。经历多伸展阶段(不同伸展方向)的大尺度物理模拟实验表明,早期伸展阶段形成的断层数目及断层的规模(长度,垂向断距)是控制断层活化的关键因素:早期伸展量越大,早期形成断层密度及规模越大,在晚期伸展阶段,越容易发生断层活化;若初始阶段伸展量有限,则新生断裂可能受先存断裂影响作用也有限,先存断裂可在局部发生活化,而整体上活化作用不明显。通过与实验结果对比,发现Turkana坳陷整体上(中生代)先存断裂活化现象不明显(活化仅发生在局部),推测研究区中生代裂谷时期伸展作用有限,导致中生代裂谷时期形成的“间隔型”先存断裂数量及规模可能有限,并且先存断裂在新生代裂谷活动中不发生明显活化,仅在局部可见活化的断层与新生断层连接,形成新生代裂谷中局部较复杂的“Z”字型断裂展布特征。小尺度砂箱物理模拟实验分析表明:(1)简单剪切模式下,凹陷整体表现为强烈的非对称半地堑构造样式,沉降中心稳定分布于单侧主边界断层附近。纯剪切伸展模式下,凹陷呈现出对称地堑构造样式,伴随伸展作用的加强,在凹陷内部形成新的中央地堑,沉降中心较稳定分布于凹陷的中央部位;(2)伸展剪切模式的转换改造凹陷构造特征,影响凹陷沉积中心迁移,随着伸展模式的转变,晚期断裂的发育特征及凹陷沉积中心的迁移特征均发生明显的响应;(3)伸展方向的转换形成凹陷斜坡带雁列式断裂和小型断陷,裂谷演化晚期,伸展方向的转换也对新生断层的发育和沉积中心的分布具有影响和控制作用。Turkana坳陷整体动力学模式在裂谷演化过程中可能发生了转换,即中新世之前简单剪切模式向中新世之后纯剪切模式的转换。Turkana坳陷新生代裂谷整体发育在坦桑尼亚克拉通东部边缘、新元古代末泛非事件形成的莫桑比克造山带上,沿前寒武系变质岩基底先存构造发育。中生代裂谷活动形成的NW向展布、下地壳尺度的“透入型”先存构造软弱带控制了Turkana坳陷整体“宽裂谷”的结构形态,中生代裂谷时期形成的“间隔型”先存断裂影响新生代裂谷局部断裂特征。研究区复杂的地幔活动,影响深部热结构,受深部热结构的控制,进一步影响裂谷的发育模式,形成不同的伸展剪切模式,控制裂谷构造演化。整体上,Turkana坳陷的发育特征和演化过程受不同类型的先存构造和地幔活动变迁的控制。更多还原

【Abstract】 The East African Rift System(EARS)is a natural modern example of active continental rift and provides ideal opportunities to study the formation and evolution of such tectonic systems.Within the EARS,the Turkana depression,the area of interaction between the Kenyan and Ethiopian rifts,is of special interest since it has experienced a complex and long-lived tectonic history,characterized by multiple stages of rifting.Previous studies generally believe that the East African Rift is controlled by the the N-S trending pre-Cambrian basement pre-existing fabrics,and develops along the paleotectonic belt.However,the faults in some areas intersect with the pre-existing fabrics of the basement on the outcrop at a large angle,showing the contral effect of the pre-existing fabrics is not so obvious.Recent studies have shown that the area has experienced at least two phases of extension.The morphology and evolution of the Cenozoic rift is obviously affected by the reactivation of the Mesozoic rift.Affected by the pre-existing fabrics of the Precambrian basement(metamorphic rock lineage and/or foliation)and Mesozoic rifting,the evolution process of Cenozoic rifts in the study area is complex,and the influence of pre-existing fabrics/faults on the development and evolution of Cenozoic rifts,on the control effect of structural morphology is still unclear.In addition,the mantle plume activity in the study area is also an important factor affecting the formation and evolution of Cenozoic rifts.It is unclear whether and how the mantle activity in the study area affects the development and evolution of Cenozoic rifts.This paper uses a large amount of two-dimensional seismic data and drilling data to conduct a comprehensive analysis of the structural geometry and kinematics of the Cenozoic rift.On this basis,the use of structural physical simulation analysis methods is used to further study the following two issues: First,In the rift activity that has undergone multiple extension stages,the characteristics of the reactivation of pre-existing fabrics(pre-existing faults)and the influence of pre-existing fabrics on the formation and evolution of later rifts;second,how does the mantle activity affect the special geometry and evolution of rifts process? How does the combined effect of mantle activities and pre-existing structures affect the evolution mode and formation mechanism of the rifts?Through the carefully interpretation of a large amount of seismic data,this paper has further deepened the understanding of the tectonic and kinematics of the Turkana Depression in the East African Rift.This study identified basic filling and structural characteristics of the 7 sags in the study area: The study area is filled with OligocenePleistocene as a whole,controlled by boundary faults,and There are five types of sag structures: typical continuous graben,typical half graben,migratory half graben,dish type graben and parallel transformation graben.From south to north,the stratum filling of sags gradually changes from old to new.In terms of structural features,there are changes from "half graben" to "graben",from "east-west parallel" NNW trendingwide rift zone to "south-north belt" narrow rift transition.According to the most recent dating data,combined with a part of the drilling and outcrop data,the rift activity began at ～ 45 Ma in the South Lokichar sag,the central Turkana Depression,and then dispread in a “leap-forward” north-south direction.By analyzing the fault activity rate of the main boundary faults and the fabrics development history of the key section,the evolution of the Cenozoic rift is divided into two periods and four evolution phases.The characteristics of the evolution of the study area are summarized as follows: early stage rift began in the south and late stage in the north,first wide rift and later narrow rift,moving to the eastward.That is,in the early Cenozoic,a series of rifts were NW-SE trends and nearly parallel;in the early Miocene,the rifts were concentrated near South Lokichar and Kerio in the south;after the Miocene,the rifts migrated northward;since the Pliocene,the rift system evolved and migrated eastward.Analogue model experiments with multiple extension phases(different extension directions)show that the number of faults formed in the early phase and the size of the faults(length,vertical fault distance)are the key factors that control fault reactivation:The greater the amount of extension,and the larger the density and scale of the faults in early stage,the easier it is for fault to be reactivated.If the amount of extension in the initial stage is limited,the effect of the pre-existing faults on the newborn faults may be limited.Pre-existing faults can be reactivated locally,but the reactivation effect is not obvious on the whole.Comparison with experimental results,it is found that the reactivation of pre-existing faults in Turkana Depression as a whole is not obvious(the activation only occurs locally).It is speculated that the extension of the Mesozoic rift in the study area is limited in the period of the Mesozoic rift.The number and scale of the existing faults may be limited,and they will be insignificantly reactivated in the Cenozoic rift period.Only the reactivated faults are connected locally with the newborn faults,forming a local complex "Z"-shaped fracture development in the Cenozoic rift.The analysis of small-scale sandbox analogue simulation experiments suggest that:(1)Under the simple shear mode,the sag generally appears as an asymmetric half graben structure,and the depocenter is stably distributed near the unilateral boundary fault.Under the pure shear extension mode,the overall structural style of the sag is a symmetrical graben structure.Together with the strengthening of extension,a new central graben is formed in the depression,and the depocenter is more stable in the center of depression.(2)The transformation of extensional shear mode has an important influence on the transformation of sag structure and the migration of sedimentary centers.The transformation of extensional mode has a obvious influence on the development characteristics of late faults and the migration characteristics of depocenter.(3)The transformation of the extension direction yield the echelon faults and small fault depressions in the depression slope belt.In the late stage of the rift evolution,the conversion of extension direction also has an influence and control on the development of newborn faults and the distribution of depocenter.The overall dynamical model of the Turkana Depression may have converted in the evolution of the rift,that is,the simple shear mode before the Miocene changed to the pure shear mode after the Miocene.The Cenozoic rift in the Turkana Depression is generally developed in the eastern margin of the Tanzania Craton,on the Mozambique orogenic belt formed by the PanAfrican event in the late Neoproterozoic,and along pre-existing fabrics of the preCambrian metamorphic basement.The NW trends and the “pervasive” pre-existing structural weakness zone at the lower crustal scale formed by the activities of the Mesozoic rift controlled the overall “wide rift” feature of the Turkana Depression.The pre-existing faults of “discrete” formed during the Mesozoic rift period affect the local fault characteristics of Cenozoic rift.The complex mantle activity in the study area affects the deep thermal structure,and is controlled by the deep thermal structure,which further affects the dynamic mode of the rift,forming different extensional shear modes,and controlling the evolution of the rift.On the whole,the development characteristics and evolution process of the Turkana Depression are controlled by different types of preexisting structures and changes in mantle activities.更多还原